The present invention relates to a method and apparatus for evacuating a receptacle using a pump arrangement consisting of a backing pump connected to the receptacle via a first valve and a fine or high vacuum pump, preferably a diffusion pump, connected in parallel with the backing pump to the receptacle via a second valve and a cold trap.
When generating a vacuum it is known to employ a plurality of differently operating pumps that are in each case particularly suitable for specific pressure ranges. Evacuation is then carried out by initially using a first pumping stage, e.g. a backing pump, to generate a pressure that lies at the upper limit of the operating range of a following pumping stage. The following pumping stage, e.g. a fine vacuum pump, is then connected, and the first pumping stage is generally disconnected. A high vacuum pump may also follow as a third pumping stage.
A known example of a multistage pumping arrangement consists of a single-stage or two-stage rotary pump cooperating with a diffusion pump. The operating range of diffusion pumps is generally below 0.2 mbar. It is therefore necessary to evacuate to a pressure in the diffusion pump operating range using the backing pump before switching over to the diffusion pump. However, it may take a relatively long time to reach this pressure since the outgassing from the internal surfaces of the vacuum system already plays a considerable role in this pressure range. In this connection, there are generally present condensable gases which condense at elevated pressures on the surfaces and are retained thereon by absorption forces. At higher pressure ranges, i.e. above 50 mbar, outgassing therefore hardly plays any role, with the result that the pressure behavior, when plotted logarithmically against time, corresponds to a straight line. However, below 50 mbar the outgassing increasingly influences the pressure behavior, with the result that the logarithm of pressure vs. time deviates from a straight line after prolonged periods. In addition, the efficiency of rotary pumps is reduced in this pressure range.
It is known to connect a cold trap in front of the high vacuum pumps, in particular the diffusion pumps, and to maintain the cold trap at a relatively low temperature, for example at the temperature of liquid nitrogen. This cold trap is not only intended to prevent oil vapors from the diffusion pump from penetrating into the vacuum system, but over and above this has a high pumping capacity for gases that condense, for example at the temperature of liquid nitrogen, with the result that the condensable gases, e.g. water vapor, present in the vacuum system are suctioned off by this condensation pump. The result is that in the pumping regions in which outgassing from the internal surfaces already plays a role, the pressure between the cold trap and the diffusion pump is lower than the pressure in the receptacle.
When using a cold trap, the transition from the backing pump to the subsequent fine or high vacuum pump may thus be effected after a fairly short pumping time, and accordingly the pumping time would be reduced as a result of the earlier connection of the fine or high vacuum pump. Such a method is especially of great advantage if, when carrying out hermetic sealing tests, a test chamber must be evacuated as quickly as possible, and in fact down to a pressure that is so low that the hermetic sealing tests can be carried out using mass spectrometers, which require a pressure of less than 10.sup.-4 mbar.
In this context it would therefore be desirable to increase the value of the pressure at which the switch-over is carried out in order to obtain the described advantage. However, this is generally not possible, since the afore-mentioned deviation of the actual pressure variation during evacuation from a straight line is not reproducible in the pressure ranges in which outgassing plays a considerable role. The pressure variation during evacuation depends on many parameters, such as atmospheric moisture, cleanliness of the system and test specimen, surface area, temperature, and the like. The possible reduction in the total pumping time by using a cold trap was therefore not previously exploited by earlier connection of the fine or high vacuum pump.